Laminate Structure Comprised of Stainless Steel Foil, Resin, and Metal Foil

ABSTRACT

The present invention provides a laminate structure with little warping comprised of a stainless steel foil, a resin, and a metal foil, that is, a laminate structure comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil wherein the stainless steel foil is comprised of mixed phases of a ferromagnetic phase and a nonferromagnetic phase and the ratio of the ferromagnetic phase is 0.1 mass % to 4.0 mass %.

TECHNICAL FIELD

The present invention can be utilized for a structure for electronic equipment such as a memory device or circuit board. In particular, it is suitable for a laminate structure for a suspension for a hard disk drive where stable positioning precision is required.

BACKGROUND ART

Composite materials, in particular laminate structures comprised of a metal and resin, give a number of properties not obtainable by single materials, so are being utilized in various fields. Their fields of utilization are increasingly growing including light and thin applications.

In these fields, to bring out magnetic, electrical, or dielectric physical actions in devices with a good precision, strict positioning precision is sought from the laminate structures supporting the devices. For example, as structures utilizing magnetic physical actions, the support parts for magnetic heads for hard disks may be mentioned, as structures utilizing electrical physical actions, flexible printed circuit boards may be mentioned, and as structures utilizing dielectric physical actions, the support parts for heads for ferroelectric memories, etc. may be mentioned.

Laminate structures are utilized in various fields, but the effect of warping can no longer be ignored due to the differences in heat expansion and heat shrinkage in each material. In particular, the effect becomes greater as the thicknesses are made smaller.

The method used to suppress warping of laminate structures has been, in the case of a thermoplastic resin, to raise the temperature to soften the laminate structure and further apply pressure to it by a press etc. to correct the warping, then to cool the resin to raise the viscosity and improve the bonding strength. On the other hand, in the case of a heat curable resin, the practice has been to apply pressure to the laminate structure by a press etc. to correct the warping and raise the temperature at that time to cure the resin.

These measures are effective against warping in the state of a high temperature of the resin, but when lowering the temperature down to near room temperature, warping occurs due to heat shrinkage. Even if eliminated, correction of the warping would require long heating and pressing by a press. This would make the productivity of the device drop remarkably.

The prior art relating to a laminate structure of a suspension for a hard disk drive, for example, Japanese Patent Publication (A) No. 2004-303358, alludes to the heat expansion coefficient and bonding strength of a resin layer, while Japanese Patent Publication (A) No. 2005-125588 alludes to the bonding strength of a resin and etchability, but, as explained above, so long as a process of heating in order to increase the bonding strength is involved in the process of production of a laminate structure, it is difficult to stably reduce the warping. The art described in the present invention, that is, the optimization of the ratio of the ferromagnetic phase inside the stainless steel foil suppressing the warping of the laminate structure, is not alluded to at all.

DISCLOSURE OF THE INVENTION

The present invention has as its object the provision of a laminate structure comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil and suppressed in warping.

The gist of the present invention lies in a laminate structure comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil, the laminate structure characterized in that the stainless steel foil is comprised of mixed phases of a ferromagnetic phase and a nonferromagnetic phase and the ratio of the ferromagnetic phase is, by mass, 0.1% to 4.0%. More preferably, it is a laminate structure for a hard disk suspension comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil, the laminate structure characterized in that the stainless steel foil is comprised of mixed phases of a ferromagnetic phase and a nonferromagnetic phase, the ratio of the ferromagnetic phase is, by mass, 0.1% to 4.0%, and the metal foil is a copper foil or a copper alloy foil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cross-section showing the configuration of the laminate structure of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the present invention will be explained more specifically. FIG. 1 is a cross-sectional schematic view showing the configuration of the laminate structure of the present invention. As will be understood from FIG. 1, the laminate structure 1 of the present invention forms a three-layer structure comprised of a stainless steel foil 2, a metal foil 3, and, between them for bonding the two foils, a thermoplastic resin or heat curable resin or a composite of a thermoplastic resin and heat curable resin 4.

The metal foil 3 is a foil of a metal and/or alloy of a composition different from the stainless steel foil 2.

This laminate structure 1 is prepared by bonding the stainless steel foil 2 and the metal foil 3 different from that by joining them by a thermoplastic or heat curable resin 4, then bonding the two by heat. The bonding temperature by this heat is generally 100 to 400° C., but the invention is not limited to this. It is possible to suitably select this by the curing or plasticizing temperature of the resin used.

In this regard, if laminating the stainless steel foil and metal foil different in composition from it forming the present invention by the interposition of a resin, warping occurs in the laminate structure along with the difference in amounts of heat shrinkage.

Stainless steel foil is comprised of a ferromagnetic phase or a nonmagnetic phase or the two phases. The ferromagnetic phase has a small amount of heat shrinkage. For example, if laminated with copper or another metal, there is a tendency for warping at the stainless steel side. On the other hand, the nonmagnetic phase has a large amount of heat shrinkage. If laminated with copper or another metal, there is a tendency for warping at the non-stainless steel side.

Since the ferromagnetic phase of a stainless steel foil has a small amount of heat shrinkage, while the nonmagnetic phase has a large amount of heat shrinkage, it is possible to adjust the ratio of these phases to control the warping at the time of making a laminate structure. In the present invention, the ratio of the ferromagnetic phase is made, by mass, 0.1% to 4.0%. If the ratio of the ferromagnetic phase is less than, by mass, 0.1%, the effect of suppressing warping is small and the structure warps at the side of the stainless steel foil, while if over 4.0%, it easily warps at the metal foil side. Note that the extent of the warping differs depending on the type of the metal foil of the laminate structure, but it is possible to suitably select the ratio within the above range to suppress warping of the laminate structure.

The method of measurement of the ferromagnetic phase is not particularly limited. A vibration sample magnetometer (VSM), ferrite meter, etc. may be utilized.

Note that the stainless steel foil described in the present invention means an austenitic stainless steel or two-phase stainless steel. Stainless steel becomes a two-phase structure by part of the nonmagnetic phase dielectrically transforming to the magnetic phase in the processing of rolling to make it thinner. The ratio of the ferromagnetic phase of the stainless steel foil can be controlled by the reduction rate at the time of rolling, the temperature of the stainless steel at the time of rolling, etc.

The “metal foil” described in the present invention is a general name for a metal and an alloy different from the stainless steel foil in ingredients and having a thickness of 100 μm or less. In particular, gold, silver, or copper has ductility and is easy to work thinly and, further, is high in electrical conductivity, can be etched to form circuits, further is superior in heat conductivity as well, and is superior in heat radiating action. Further, alloying enables the improvement of the mechanical strength.

For copper alloys in the metal foils, as alloying elements, typically Ni, Si, Mg, Be, etc. are used, but the invention is not limited to these. From the viewpoint of maintaining physical properties substantially the same as Cu alone etc. (other than mechanical strength), it is believed that Cu is preferably 90 mass % or more.

The resin described in the present invention includes both a thermoplastic resin and a heat curable resin. The “thermoplastic resin” indicates a thermoplastic polyimide, polystyrene, polyethylene, polyamide, etc., but the invention is not limited to these. In particular, in the case of a polyamide superior in heat resistance, the amount of heat shrinkage between room temperature and a high temperature is large and the effect of suppression of warping described in the present invention is large.

The “heat curable resin” indicates a heat curable polyimide, urea resin, melamine resin, phenol resin, epoxy resin, unsaturated polyester, alkyd resin, urethane resin, ebonite resin, etc., but the invention is not limited to these. The present invention exhibits its effect by all resins requiring a heating step. The resin may be a single layer or may be a plurality in a laminar state. The selection and combination of the thermoplastic resin layer and heat curable resin layer may be any selection and combination. The thickness of the resin (in the case of multiple layers, the total) is preferably 5 μm to 100 μm, for the purpose of reducing the weight, more preferably 5 μm to 25 μm, but the invention is not limited to these values.

In the recently fast growing field of hard disk drives, the increasingly higher densities are being accompanied with an increasingly narrower distance between the magnetic head and recording medium with each passing year. The distance of 100 nm around 1995 is currently being made a closer 20 nm or less. On the other hand, the suspensions for supporting magnetic heads are being required to be reduced in weight due to the vibration resistance characteristics. Stainless steel with such a required thickness of 100 μm or less (generally, a thickness of 100 μm or less is called a “foil”) is being used.

According to the present invention, it becomes possible to eliminate the warping of a laminate structure comprised of a stainless steel foil, a resin, and a metal foil and possible to stably produce a laminate structure for a hard disk drive suspension with little of the problems of positioning precision due to warping caused by the difference in shrinkage even if thin.

Examples

Below, the present invention will be explained in detail based on the examples.

Example 1

A stainless steel foil comprised of SUS304 foil with a thickness of 20 μm and a width of 400 mm and copper foil with a thickness of 20 μm were bonded by curing a thermoplastic polyimide resin (curing temperature: 350° C.) and the state of warping was examined. For the warping, the laminate structure was cut to 1 m length, then the top end was fixed and the structure was hung downward. A maximum amount of warping of the laminate structure from the vertical plane of 100 mm or less was judged to be good. The thickness of the thermoplastic polyimide resin was made 5 μm, 25 μm, and 100 μm.

The stainless steel foils used for the samples good (low) in terms of warping and samples not good in it were measured for saturated magnetic flux density by a vibration sample magnetometer (VSM), whereupon it could be confirmed that if a ferromagnetic phase is present in the nonmagnetic phase in a certain range, a good laminate shape is obtained.

A vibration sample magnetometer (VSM) is expensive, has to be set in a clean environment, and is expensive in terms of maintenance and inspection costs as well, so a ferrite meter able to simply measure the ferromagnetic phase was used for measurement. The results are shown in Table 1. For the measurement by the ferrite meter, the stainless steel foil was stacked to 1.0 mm and brought into contact with the magnetic sensor part for measurement. The measurement temperature of the vibration sample magnetometer (VSM) and ferrite meter was made room temperature (25° C.).

Note that the measured value of the saturated magnetic flux density by a vibration sample magnetometer (VSM) is small in dependency on the dimensions and shape of the sample, while the measured value of the ratio of the ferromagnetic phase by a ferrite meter has a shape dependency for the thickness of the sample from the measurement principle. For this reason, the measurement by the ferrite meter in the present invention was standardized to the condition of a sample thickness of 1.0 mm obtained by stacking the stainless steel foil to a sample thickness of 1.0 mm.

By making the sample thickness of the laminated foil constant, the saturated magnetic flux density of the stainless steel foil and the ferromagnetic phase ratio by measurement by the ferrite meter have a constant correspondence as shown in for example Table 1.

TABLE 1 Saturated magnetic flux Ferrite Resin density Bs meter thickness Sample (G) (mass %) (μm) Laminate shape Sample 1 30 or less 0.1 or less 5 Warping at stainless (0.095) steel side Sample 2 25 Warping at stainless steel side Sample 3 100 Warping at stainless steel side Sample 4 370 0.5 5 Shaped well Sample 5 25 Shaped well Sample 6 100 Shaped well Sample 7 490 1.0 5 Shaped well Sample 8 25 Shaped well Sample 9 100 Shaped well Sample 10 630 1.4 5 Shaped well Sample 11 25 Shaped well Sample 12 100 Shaped well Sample 13 1320 4.0 5 Shaped well Sample 14 25 Shaped well Sample 15 100 Shaped well Sample 16 1850 6.0 5 Warping at copper side Sample 17 25 Warping at copper side Sample 18 100 Warping at copper side

Example 2

A stainless steel foil comprised of SUS304 foil with a thickness of 100 μm and a width of 650 mm (however, in the case where the metal foil is Au or Ag, a width of 20 mm) and various types of metal foil (thickness of 100 μm and width of 650 mm, however when the metal foil is Au or Ag, a width of 20 mm) were bonded by curing an epoxy resin (resin thickness 18 μm) and the state of warping (curing temperature of 100° C.) was examined.

Note that the copper alloy Cu—Ni—Si—Mg used was one of a range of composition of Ni: 2.2 to 4.2 mass %, Si: 0.025 to 1.2 mass %, and Mg: 0.05 to 0.3 mass %.

Further, the warping was examined in the same way as Example 1.

The stainless steel foil was annealed at 1050° C., then rolled until 100 μm. The ratio of the magnetic phase was measured while changing the reduction rate at that time. In the measurement, the stainless steel foil was stacked to a thickness of 1.0 mm and measured by a ferrite meter.

The ratio of the magnetic phase of the stainless steel foil optimal for each type of metal foil is shown in Table 2. By optimizing the ratio of the magnetic phase, a laminate structure free of warping is obtained.

TABLE 2 Mass ratio (%) of magnetic phase in stainless Type of steel foil metal foil 0.1% 0.5% 1.0% 2.0% 3.0% 4.0% 6.0% Au Δ Δ Δ ◯ ◯ ◯ ⋆ Ag ◯ ◯ ◯ ⋆ ⋆ ⋆ ⋆ Cu Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Al ◯ ⋆ ⋆ ⋆ ⋆ ⋆ ⋆ Mg ◯ ⋆ ⋆ ⋆ ⋆ ⋆ ⋆ Sn Δ Δ Δ ◯ ◯ ◯ ⋆ Cu—Ni—Si—Mg Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Cu—Ni Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Cu—Si Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Cu—Mg Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Cu—Be Δ ◯ ◯ ◯ ◯ ⋆ ⋆ Δ (Poor): Case of warping at stainless steel side ◯ (Good): Shaped well ⋆ (Poor): Case of warping at metal side

Example 3

A stainless steel foil comprised of various types of stainless steel foil with a thickness of 20 μm and a width of 300 mm and a copper foil (thickness of 20 μm and width of 300 mm) were joined by curing a thermoplastic polyimide resin (resin thickness of 10 μm) (curing temperature: 350° C.) and the state of warping was examined.

Note that the warping was examined in the same way as in Example 1.

Each stainless steel foil was annealed at 1150° C., then rolled to 20 μm. The ratio of the magnetic phase was measured while changing the reduction rate at that time. In the measurement, the stainless steel foil was stacked to a thickness of 1.0 mm, then measured by a ferrite meter.

The ratio of the magnetic phase optimum for each stainless steel foil is shown in Table 3. By optimizing the ratio of the magnetic phase, a laminate structure free of warping is obtained.

Here, the type of steel of the stainless steel foil is one based on JIS G4304 and JIS G4305. Further, these steel types are linked with the UNS, AISI, DIN, etc. of the ISO and related foreign standards.

TABLE 3 Mass ratio (%) of magnetic phase in stainless Type of steel foil stainless steel 0.1% 0.5% 1.0% 2.0% 3.0% 4.0% 6.0% SUS301 Δ ◯ ◯ ◯ ◯ ⋆ ⋆ SUS302 Δ ◯ ◯ ◯ ◯ ⋆ ⋆ SUS303 Δ ◯ ◯ ◯ ◯ ⋆ ⋆ SUS304 Δ ◯ ◯ ◯ ◯ ⋆ ⋆ SUS316 ◯ ◯ ◯ ⋆ ⋆ ⋆ ⋆ SUS316L ◯ ◯ ◯ ⋆ ⋆ ⋆ ⋆ SUS317 ◯ ◯ ◯ ⋆ ⋆ ⋆ ⋆ SUS321 Δ Δ ◯ ◯ ◯ ◯ ⋆ SUS347 Δ Δ ◯ ◯ ◯ ◯ ⋆ Δ (Poor): Case of warping at stainless steel side ◯ (Good): Shaped well ⋆ (Poor): Case of warping at copper side

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to stabilize the warping of a laminate structure comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil at a low level. Specifically, the invention can be utilized for a wide range of fields where strict positioning precision is demanded such as the support of a magnetic head for a hard disk, a flexible printed circuit board, a support of a head for a ferroelectric memory, or the support of another electrical, magnetic, or dielectric device. Inn particular, the invention can become an effective means in suspension members for hard disk drives where a thin structure and a high precision are demanded. 

1. A laminate structure comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil, said laminate structure characterized in that the stainless steel foil is comprised of mixed phases of a ferromagnetic phase and a nonferromagnetic phase and the ratio of the ferromagnetic phase is, by mass, 0.1% to 4.0%.
 2. A laminate structure for a hard disk suspension comprised of a three-layer structure of a stainless steel foil, a resin, and a metal foil, said laminate structure characterized in that the stainless steel foil is comprised of mixed phases of a ferromagnetic phase and a nonferromagnetic phase, the ratio of the ferromagnetic phase is, by mass, 0.1% to 4.0%, and the metal foil is a copper foil or a copper alloy foil. 